What source were they playing? For instance, the Spotify Connect software maintains control of your speakers for a user even after they’ve left your own Wi-Fi. 
 

Have you tried to call Sonos Support directly to discuss it, particularly with your movie?

Thanks for responding!

My own music ....I don't generally stream Spotify and the like. The thing that gets me is that no app was open for Sonos…

I haven't got in touch with Sonos directly yet... thought I'd see what others had for ideas.

Bizarre…

Thanks again

If it happens again make a diagnose within twenty minutes and contact Sonos. They will be able to see what’s happened. On this community I’ve seen Spotify Connect issues and cats or even lizards triggering Sonos speakers.

Lol! Nah that won't be it....think I'd have a giggle

Appreciate your help...I will do as you recommend. And hopefully it won't happen when I'm sound asleep and scare the living daylights out of me!

I can well imagine. It’d be frightening for me, too.

There’s only one case I’m aware of in which ‘local’ music can get started, and it requires some sort of either hardware failure in the physical button, or an animal accidentally clicking on a ‘resume’ function. My cat used to love the extra heat generated by my old PLAY:5s, and would sometimes sit on top of it. Every once in a while, she would hit a button as she got down, until I went in to the controller and turned off the hard buttons on the speaker. Of course, as soon as the speaker would start playback, she’d scamper away, and it was hard to make that connection initially.

So, a couple of things. First, I’d definitely go in to the controller and turn off any of those buttons that may be still on. And second, @106rallye idea about getting that diagnostic within 10 minutes should reveal what initiated that playback command. Just need to submit that to Sonos, and call in. 

And, as a random thing, I’d highly recommend that you retire the BRIDGE. Either replace it with the newer BOOST, wire one of your speakers directly, or put the whole system on your Wi-Fi. That BRIDGE is prone to failure at this point..